The present invention relates to a method and a device for determining the actual capacity of a battery by using a battery characteristic.
In a motor vehicle, wherein a battery (=rechargeable electrical energy source; secondary battery or accumulator) is used as the sole or as an additional energy source aside from an internal combustion engine for driving the vehicle (electric vehicle, hybrid vehicle), the capacity of the battery plays an important role for the range of the motor vehicle to be travelled by the vehicle without recharging the battery.
It is known that the removable amount of energy, i.e. the removable amount of current, of for example a conventional lithium-ion battery decreases with increasing age and/or the number at charging and discharging cycles.
This loss of capacity, commonly referred to as aging and representing a manifestation of the change in the behavior of batteries during charging and discharging (when used as intended) or due to time-related aging, can presently only be inadequately determined.
The known methods for determining the loss of capacity are either very complex and/or are difficult to parameterize (e.g. with impedance measurements) or the employed aging models are not completely or only inadequately parameterized, which can cause significant deviations in the calculation.
Simple methods have usually the disadvantage that important parameters cannot be considered or are simply also not known (e.g. the power loss or efficiency, which need not be constant over the lifetime and the use the batteries).
An unambiguous statement about the current energy content, i.e. the currently available amount of current, can therefore not easily be made during an ongoing or normal operation. Because the battery capacity is reduced due to aging, any conclusion concerning the availability will fail or be inaccurate when the actual (or corrected) battery capacity is not properly tracked.
Many known methods for determining the actual capacity of a battery require the determination of charge states (State of Charge, SOC) of the battery, based on measurements of open circuit voltages (OCV) of the batteries, as well as the measurement of the amount of current removed between two charge states.
For example, EP 1 314 992 A2 describes a device and a method for determining the capacity of a battery for propelling an electric car, wherein a first charge state of a battery and a second charge state of a battery are determined by way of the respective open circuit voltages in the load-free state and the actual capacity of the battery is determined from the amount of discharge current flowing between the two charge states.
DE 10 2007 037 041 A1 discloses a method and a device for detecting the battery state of an acid battery by using a stored battery model. The open circuit voltage of the acid battery as a function of the charge that can be removed from the battery is stored in form of a characteristic curve. A first parameter is then determined, which describes the loss of battery capacity due to aging of the battery and an acid stratification of the battery. Furthermore, a second parameter is determined which provides information about the slope the characteristic curve. Moreover, the stored characteristic curve is adapted by taking into account the first and second parameter.
According to this document, the removable amount of energy and the actual capacity of an acid battery are determined by determining an open-voltage-ampere-hours characteristic curve. In comparison to a new battery without acid stratification, the characteristic curve of an aged battery is displaced upward in a parallel fashion, whereas the characteristic curve of a battery with acid stratification has an increased slope. The open-voltage-ampere-hours characteristic curve can be determined by measuring the open-circuit voltage of the battery before and after a discharge. The stored characteristic curve is then adapted by using the actually determined voltage-ampere-hours characteristic curve.
DE 10 2010 006 965 A1 describes a method for determining at least a range of a battery characteristic curve for a battery in a vehicle, wherein the actual charge state of the battery is determined from measured battery parameters, an adjustment value is then determined from the difference between a measured battery parameter and a standard value, which is determined from a standard battery characteristic curve by taking into account the determined charge state, and at least one range of a new battery characteristic curve is determined by adding the adjustment value to the standard battery characteristic curve.
According to this document, the actual battery characteristic curve is determined by measuring of open-circuit voltages after a longer resting phase of the battery at different charge states of a battery. The deviation of the actual battery characteristic curve (open-circuit voltage-actual charge state characteristic curve) from the battery characteristic curve of a “standard battery” is determined based on a comparison of the respective open-circuit voltages for specified charge states or based on a comparison of respective charge states for specified open-circuit voltages.
A computing device is known from JP 2002-243 813 A, which calculates a change in the charge state of a battery from the time-integrated discharge current of the battery based on the open-circuit voltages at the start time of the integration and the end time of the integration, as well as the correlation between the open-circuit voltages and the charge state of the battery. The computing device also calculates the diminished capacity of the battery over time based on an integrated discharge current value calculated based on the discharge current and the change of the charge state. In addition, the computing device calculates the capacity degradation rate based on the diminished capacity over time and the initial capacity of the battery.
DE 39 10 904 A1 discloses a method for monitoring the charge state of a rechargeable, sealed battery, wherein the voltage UKL at the battery terminals is measured after charging a battery—following at least one load—and stored as open-circuit voltage U0, and wherein at different times the instantaneous voltage Uti is measured at the battery terminals and the discharge state is determined as a function of the difference between the open-circuit voltage U0 and the instantaneous voltage Uti.
As can be seen from this short overview, all the above described methods and devices require the measurement of an open-circuit voltage (OCV) of the battery. However, this open-circuit voltage can be measured with sufficient accuracy only after a not insignificant amount of time has passes after the end of a discharge. As is evident, for example, from FIG. 6 of EP 1 314 992 A2, a resting phase of up to 3000 seconds, i.e. of up to 50 minutes, may be required before the voltage reaches a stable value following the termination of a discharge process.